Field of the Invention
The present invention relates to a method for improving solar energy conversion efficiency using metal oxide photocatalysts having an energy band of core-shell structure for ultraviolet (UV) ray and visible light absorption, and photocatalysts thereof. More particularly, the present invention relates to a method for improving solar energy conversion efficiency including: a first process of performing heat treatment on a metal oxide semiconductor having a band-gap to form a nanoparticle thin film layer; a second process of contacting a plasma ball including mixed gas in a substitutional —NH or NHx radical state by a plasma reaction under a hydrogen and nitrogen gas atmosphere with a surface of a metal oxide particle to simultaneously generate a NH functional group and oxygen vacancies formed by hydrogenation, so as to prepare a core-shell metal oxide capable of absorbing UV ray and visible light by a single process at room temperature within 3 minutes; and a third process of further depositing a transition metal on surfaces of core-shell metal oxide nanoparticles to produce a photocatalyst of HN-metal oxide having a HN-core-shell structure for energy conversion.
In the above processes, a valence band maximum (VBM) level that can be occupied by electrons is raised by binding a —NH functional group to metal moiety of the metal oxide to increase an energy band of the metal oxide, thus enabling absorption of visible light. In addition, oxygen vacancies may be generated from the metal oxide using hydrogen, thus enabling the electrons excited on a core part of the metal oxide under UV ray to move into holes present in the shell. As a result, the core-shell metal oxide may be used to considerably increase amounts of the electrons and holes excited in a shell region under UV ray and visible light, thus controlling the energy band.
The production of a core-shell metal oxide through an oxygen vacancy formation process using —NH and hydrogen proposed in the present invention may be applied to a broad range of metal oxide semiconductor photocatalyst materials including, for example, TiO2, ZnO, CuO, etc. In addition, any material treated by the above processes may have characteristics capable of: 1) extending a range of absorbable light wavelengths, 2) increasing a major carrier density, 3) enabling fast transfer of electron-hole pairs excited by light energy to an outside before these are recombined and disappear, and 4) improving overall oxidation/reduction reaction characteristics of the metal oxide, and thereby, it is possible to employ the core-shell metal oxide in a broad range of applications including not only solar energy conversion catalysts of carbon dioxide but also various fields based on the metal oxide semiconductor such as production of solar cells, disinfection of bio-microorganisms, and the like.
Description of the Related Art
Carbon capture and utilization (CCU) is an advanced eco-friendly energy circulation technique, which is based on the photosynthesis principle in nature to produce different hydrocarbon compounds such as carbon monoxide (CO), methane (CH4), methanol (CH3OH), formic acid (HCOOH), etc. by using solar energy, water and CO2. The purpose of the CCU technique is essentially to develop a photocatalyst with the possibility of realizing an artificial photosynthesis technique that oxidizes water (H2O) with received solar energy, and at the same time, reduces CO2. The metal oxide photocatalyst including representative examples such as TiO2 or ZnO has excellent economic advantages, stability of reaction, durability and non-harmful effects on the human body and environment, compared to other catalytic materials. Nevertheless, due to very low solar energy conversion efficiency, utilization of the above technique has been limited Major reasons of the low energy conversion efficiency may be as follows: 1) due to the wide band-gap, only solar light in the UV range can be used as an energy source, therefore, only 4% of the whole solar spectrum is used at most; and 2) due to characteristics of semiconductor materials, the photogenerated electron-hole pairs cannot be reliably separated and have a high probability of recombination, this results in minimal utilization in final oxidation/reduction reactions.
In order to overcome such factors causing a decrease in efficiency as described above, a representative method may include introducing different metal and non-metal elements into a crystalline structure of existing metal oxides to vary a binding energy or charge density between constituent atoms inside the crystal, thereby reducing a size of the band-gap and enhancing an electron-hole transfer rate. However, in case of introducing metal elements, a conduction band minimum (CBM) level essential in the reduction reaction of CO2 and H2 is significantly changed. On the other hand, when introducing non-metal elements, a number of defect sites which cause an increase in the electron-hole recombination rate are generated in the crystalline structure. Therefore, each of the above treatments still requires additional complementary processes.
The present invention proposes a simple treatment method that utilizes an oxygen vacancy formation process using —NH group and hydrogen to produce a core-shell metal oxide capable of absorbing UV ray and visible light from the metal oxide, respectively, so as to improve electron-hole transfer characteristics and increase an applicable wavelength range. Meanwhile, the conventional plasma process has been executed by exposing a metal oxide material under a hydrogen gas atmosphere at a high temperature and a high pressure (200° C., 10 bar or more), for at least one day. Further, the conventional nitrogenation process has been mostly executed by treating an ammonia gas at a high temperature of at least 500° C. for 1 hour or more. However, when these two processes are separately conducted, the non-metal elements inserted into metal oxide crystals are released out of the crystals at a temperature of 300° C. or more, therefore, desired effects cannot be achieved. On the other hand, when these two processes are conducted in combination, high reactive gases are mixed at a high temperature to cause a danger of explosion, hence entailing a limitation in the practical use thereof. Korean Patent Laid-Open Publication No. 2006-0018751 discloses low temperature plasma treatment, however, this treatment is executed at 50° C. for 30 minutes. In contrast, the present invention relates to organic material degradation, which exhibits non-comparably lower efficiency compared to such a process of treating at room temperature for 3 minutes, has relatively low utility in terms of characteristics, and is easily reactive. However, the present inventive method utilizes a single process, instead of such step-by-step processes as described above, to generate —NH molecules and bind —NH to a metal atom moiety in the shell region of the metal oxide so as to increase an energy band of VMB, and thereby forming electron-holes in a visible light region. A technique capable of producing —NH in a single process, which was not generated by the conventional step-by-step process using hydrogen plasma and nitrogen plasma, has been assured, thus overcoming a limitation in transferring electron-holes onto a surface of the metal oxide under UV ray and/or visible light, which could not be attained in the conventional step-by-step process. Therefore, the above technique becomes applicable to energy conversion catalysts. In addition, the hydrogen used in the single process may generate oxygen vacancies in the shell region, thus efficiently forming electrons-holes under UV ray and/or visible light and actively transferring the electrons-holes toward the surface of the catalyst particles. As a result, a metal oxide catalyst applicable to energy conversion could be successfully manufactured.
Metal oxide nanoparticles provided by the present invention may have some characteristics capable of: 1) extending a wavelength range of absorbable light to the visible light region while preserving a required energy band level for CO2 reduction and water oxidation; 2) noticeably increasing an electron carrier density; 3) quickly transferring the electron-hole pairs excited by light energy out of the nanoparticles due to energy levels newly created in the metal oxide band-gap of the above-described core-shell structure before these are recombined and disappear; and therefore, 4) remarkably improving overall oxidation/reduction reaction characteristics of the metal oxide. Further, depositing a transition metal on the surface of such the core-shell metal oxide may enable solar energy conversion of carbon dioxide into methanol or carbon monoxide.
Although the same prior art as the present invention is not yet disclosed, some conventional arts similar thereto may be described as follows:
1) Korean Patent Registration No. 10-0950623 (a method for increasing compression stress of PECVD silicon nitride films): a technique for enhancing compression stress characteristic of a silicon-nitride coating film, which includes depositing a silicon-containing precursor on a semiconductor element by treatment of the precursor using H2 gas plasma and mixed plasma of H2 and N2 gases, in a sequential order.
2) Korean Patent Registration No. 10-1058735 (a solar cell and a method of manufacturing the same): a technique for manufacturing a solar cell electrode with enhanced passivation effects, which includes forming an insulating film having a hydrogen content of less than 10% on the surface of a semiconductor electrode through silane and ammonia mixed gas plasma treatment.
3) Korean Patent Registration No. 10-1310865 (a method and an apparatus for manufacturing a nanoparticle composite catalyst by plasma ion implantation): a technique for manufacturing a homogenized nanoparticle composite catalyst using a small amount of catalyst components, which includes injecting solid elements instantly ionized through solid element plasma ion implantation into a porous carrier substrate.
4) Korean Patent Registration No. 10-0510049 (a simultaneous desulfurization and denitrogenation method using a combination process of low temperature plasma and low temperature catalyst, and an apparatus used for the same): a technique for neutralizing nitrogen oxides and sulfur oxides contained in an exhaust gas, followed by removing the same through catalysis, which includes charging the exhaust gas in a low temperature plasma reactor filled with ammonia and propylene.
Other than the above four patent cases proposed as the representative examples, most of the conventional arts are concentrated on the improvement of physical properties, neutralization and treatment of harmful gases, and production of nanoparticles and a uniform coating film, which are substantially independent of the present invention with the purpose of improving photochemical catalytic conversion properties. Alternatively, it has been developed a process technique which includes: synthesizing a core-shell structure capable of absorbing UV ray and visible light through —NH production by a single process used in the present invention and through formation of oxygen vacancies using hydrogen; and then treating the surface of such a core-shell metal oxide material using a transition metal such as Cu, so as to develop an energy conversion catalyst. Therefore, these patents have essential differences in technical configurations, as compared to application of two different element treatment characteristics in a single process.
Further, other similar conventional arts disclosed in research papers are as follow:
1) Enhancing Visible Light Photo-oxidation of Water with TiO2 Nanowire Arrays via Cotreatment with H2 and NH3: Synergistic Effects between Ti3+ and N (J. Am. Chem. Soc. 2012, 134, 3659): a technique for hydrogenation and nitrogenation of TiO2 nanowires by sequentially treating the same with hydrogen and ammonia gases at 500° C. for 1 hour, respectively.
2) Core-Shell Nanostructured Black Rutile Titania as Excellent Catalyst for Hydrogen Production Enhanced by Sulfur Doping (J. Am. Chem. Soc. 2013, 135, 17831): a technique including heat treatment of TiO2 nanoparticles with aluminum at 800° C. for 6 hours to conduct reduction of the surface of the nanoparticles, followed by flowing H2S gas at 600° C. for 4 hours to inject a sulfur element into TiO2 while providing hydrogenation-like effects thereto.
3) Effective nonmetal incorporation in black titania with enhanced solar energy utilization (Energy Environ. Sci. 2014, 7, 967): application of various elements for implantation such as hydrogen, sulfur, iodine and nitrogen, in the same technical method as disclosed in the research paper of 2).
These published research papers describe techniques for hydrogenation of metal oxides and insertion of other non-metal elements by using reactive gases such as ammonia, hydrogen sulfide, etc. through at least two separate processes at a high temperature of at least 500° C. or more for a relatively long time of 1 hour or more. Therefore, such technical methods as described above are significantly different from the method proposed and realized by the present invention.